I believe the authors wrote it clear enough that they don't rule out UTF-16 completely: > We believe that all other encodings of Unicode (or text, in general)

I believe the authors wrote it clear enough that they don't rule out UTF-16 completely: > We believe that all other encodings of Unicode (or text, in general) belong to rare edge-cases of optimization and should be avoided by mainstream users. So if you're writing a browser that must use UTF-16 in its Javascript engine due to dumb standards... it is a reasonable performance optimizations to use UTF-16 for your strings. But how many people write Javascript engines?